Field of the Invention
This invention relates to improving the color and viscosity stability of copolymeric ethylene-.alpha.-olefin viscosity index improvers conventionally incorporated in mineral oil base lubricants.
Such lubricants when subjected to elevated temperatures in service have a tendency to become thin. To retard this tendency, it is common practise to add to the lubricants viscosity index improvers such as ethylene .alpha.-olefin copolymers.
One problem encountered with such VI improvers is their lack of thermal shear stability which is a measure of the reduction of their viscosity after prolonged storage at 180.degree.-300.degree. F in air. This type of storage also results in a darkened appearance. Such a color degradation is unacceptable from the point of view of consumer appeal.
These and other related problem have mitigated against wide acceptance of ethylene-based polymers as lubricant viscosity index improvers.